The present invention relates to a brake pad wear detector for detecting wear of brake pads of a disk brake for automobiles.
There is known a method of predicting when to replace brake pads by mounting a detector (hereinafter called probe) at a wear limit position of the brake pads to detect wear of lining of a brake pad, and electrically detecting breakage of a lead wire based on a change in voltage. The probe is easily attachable to and detachable from brake pads which are expendable. In order to accurately detect the wear limit of linings, various mounting structures have been proposed.
FIG. 4 shows a structure for mounting a brake pad wear detector proposed in Japanese utility model publication 6-22637 filed by the present applicant. A probe 101 is provided inside of the outer edge of a lining 1 of a brake pad 3, and is resiliently mounted by a resilient clip 5 in a cutout 4 formed in a back plate 2 of the brake pad 3 at its outer edge.
FIG. 5 is an enlarged perspective view of the cutout. The cutout 4, into which the probe 101 is inserted from the outer periphery of the brake pad 3 in a radial direction of the disk rotor (not shown), has straight guide rails 4a extending in the direction of thickness of the back plate 2 and slightly narrower than guide grooves 101a formed in the probe 101 (FIG. 6). A curved cutout 4b is provided at a side of the back plate 2 opposite the lining 1 and has shoulders 4c.
FIG. 6 is a perspective view of the probe 101. A lead wire 102 for detecting the wear of the lining 1 is bent in the shape of the letter U and buried in the probe 101 which is made from e.g. a resin. The guide rails 4a of the back plate 2 (FIG. 5) are adapted to engage in guide grooves 101a formed in both sides of the probe 101. When the lining 1 gets worn to some degree by the disk rotor (not shown), the lead wire 102 will be exposed and thus abraded by the disk rotor until eventually cut. When the wire is cut, a warning lamp goes on.
FIG. 7 is a perspective view of the resilient clip 5 for retaining the probe 101 inserted in the back plate 2. The clip 5 comprises two substantially U-shaped strips 5c connected together by a connecting strip 5b having projections 5a at both ends. The projections 5a engage the shoulders 4c and prevent the resilient clip 5 from dropping.
FIG. 8 is a top plan view in which the cutout 4 is viewed from above with the brake pad 3, probe 101 and resilient clip 5 assembled together. In FIG. 8, to mount the probe 101 on the back plate 2, the guide grooves 101a of the probe 101 are slid along the guide rails 4a of the cutout 4 formed in the back plate 2 to engage the back plate. Then the resilient clip 5 is pressed onto the back plate 2 of the brake pad 3 so that its strips 5c engage both sides of the plate 2. The strips 5c of the clip 5 will deflect along the tip shapes of the two projections 5a at both ends of the connecting strip 5b. When they snap in and engage the shoulders 4c of the cutout 4, they are assembled together.
In this state, the probe 101 is bound by the engagement between the shoulders 4c of the cutout 4 and the projections 5a of the clip 5, so that there is no freedom of movement in the vertical direction. Also, bound by the engagement between the guide rails 4a of the cutout 4 and the guide grooves 101a of the probe 101, there is no freedom in the groove direction. But they are formed such that clearances S (FIG. 8) are present between the back plate 2 and the side faces of the guide grooves 101a of the probe 101 in the thickness direction due to the resilience of the strips 5c of the clip 5.
The reason why the clearances S are present is described with reference to sectional views 9A and 9B in which the probe 101 in the cutout 4 is viewed from the side. FIG. 9A shows the lining 1 when it is brand-new. FIG. 9B shows a state in which the lining 1 has been worn to the wear limit X until the lead wire 102 has been cut by the disk rotor 10, so that the warning lamp is now on. As the lining 1 is abraded, the probe 101 made of resin is brought into contact with and abraded by the disk rotor 10, which is made of cast iron.
During this process, if there were no clearances S and thus there were no freedom of movement between the guide rails 4a of the cutout 4 and the guide grooves 101a of the probe 101, the lining 1 and the disk rotor 10 would collide impulsively against each other upon abrupt braking and the probe 101 and the rotor be exposed to shock, so that the probe 101, made from a weaker material, would be broken, making it impossible to stably detect the wear limit X. The clearances S serve to absorb such shocks. The probe 101 is thus abraded in a normal manner, so that the lead wire 102 is cut at a predetermined position.
In this conventional arrangement, the straight guide rails provided in the cutout of the back plate are adapted to engage in the guide grooves of the probe. The probe is snapped in and engaged with the shoulders of the cutout of the back plate, held by the resilient clip, so that it can be easily attached and detached. Since the probe is kept resiliently fastened to the disk rotor by the resilient clip, it is protected against collision against the disk rotor, so that it is possible to stably detect the wear limit.
But when expandable brake pads are repeatedly replaced, the disk rotor will become slender as shown in FIG. 10. In order that the lining is used effectively, the lining 1 of the brake pad is usually mounted so that its outer edge will be 1-2 mm radially inside the outer edge of the rotor 10. This means that the outer annular edge of the rotor does not contact the lining, so that an annular rib 10a is formed along the outer edge of the rotor 10. When the disk rotor 10 has been abraded by the lining to the depth of H, the lead wire 102 will be cut by the rib 10a even though the thickness of the liner 1 is still Y, which is larger than the wear limit X by an amount H.
If only the brake pad is replaced repeatedly with the same disk rotor, the rib 10a will increase and thus the thickness Y of the lining when the wire is cut by the rib 10a will become larger. This means that the warning lamp goes on even though the lining 1 is still sufficiently thick and usable. When the lamp goes on, an operator will check the lining. But since the lining is still sufficiently thick, he will judge that the alarm is false and keep on using the pad instead of replacing it with a new one, not knowing the fact that the disk rotor is getting dangerously thin. When the height of the rib 10a increases to a certain value, the piston will have to move a distance beyond a point at which the piston disengages from the fluid seal member to move the lining into frictional contact with the rotor. If this happens, fluid will leak.
An object of the present invention is to provide a lead wire shaped such that it will be cut at a use limit height H of the rib 10a of the rotor, or at the wear limit X of the lining 1.